Planetary gear boxes are known. FIG. 1 shows a sectional view of a known planetary gear box arrangement. The gear box comprises a sun gear 1 centred around a centreline C-C. The sun gear 1 is mounted to an input shaft 2 which is mounted for rotation in a first bearing 3. The sun gear 1 meshes with an array of planet gears 4 (only one of which is shown) arranged around the toothed circumference of the sun gear 1. A ring gear 5 encircles and meshes with the planet gears 4. The ring gear 5 is fixed in position and so, upon rotation of the sun gear 1, the planetary gears 4 are forced to rotate around their individual axes A-A and travel within the ring gear 4 and around the sun gear 1.
The planet gears 4 are each mounted for rotation on a centrifugal carrier 7 which in turn connects to an arm 6a of an output carrier 6b via a spherical bearing or pivot 7a. The arm 6a is arranged to transmit the output via an output shaft 6 which nominally shares a common axis C-C with the input shaft 2. The output shaft 6 is further mounted for rotation with respect to a static casing by means of a bearing or pivot 9. The centrifugal carrier 7 is arranged for rotation about the axis C-C by means of a bearing 8. The centrifugal carrier 7 and the output shaft 6 rotate together.
FIG. 2 illustrates the planet gear 4, the output carrier 6a, 6b and centrifugal carrier 7 in closer detail. It will be appreciated that in order to maintain gear alignment in the system, centrifugal carrier 7 must be made stiff. In particular the centrifugal carrier 7 is radially stiff and contains the centrifugal loading of the planet gears 4. However, the planet gear 4 is inevitably subjected to a torsion load T. Due to the rigidity of the carrier alignment, shear loads may be applied to the centrifugal carrier 7 and planet gears 4. These are minimised by the provision of a bearing 10 where the arm 6a engages with the centrifugal carrier 7. Typically, the bearing 10 is a spherical bearing. Bending loads act on the bearing 10 and can result in a torsional deformation of the arm 6a whose axis A′-A′ moves out of alignment with the axis A-A of the planet gear 4. Due to clearances and manufacturing tolerances amongst the carrier components, the difference between axes A′-A′ and A-A may vary between planet gears held in the centrifugal carrier 7. Consequently, as the gears 1, 4 rotate, local orbiting of the arms 6a occurs. This orbiting can result in imbalance and vibration in the transmission system and ultimately throughout the machine in which it operates. In particular, this imbalance can tend to move the output shaft 6 and input shaft 2 out of alignment along the common axis C-C.